Selective amplification can be achieved by the use of one or more amplifier stages followed and/or preceded by passive filters with a suitable band-pass characteristic. If such a filter preceded a wide-band amplifier, the latter may generate considerable noise outside the selected range. If the amplifier is followed by the filter, it may be overloaded by out-of-band signals which could result in saturation and undesirable cross-modulation products. The use of two highly selective filters, upstream and downstream of the amplifier or a stage thereof, is costly and may lead to loss of selectivity due to deviations between the pass-band characteristics of these filters.
In the case of multistage amplifiers of this type, there is also the problem of supplying operating current to the several stages from a d-c source with minimum power loss and signal distortion. Thus, supplying the operating current to each stage through an individual resistor involves considerable energy dissipation whereas the use of choke coils for this purpose introduces parasitic capacitances, e.g. between the turns of the coil.